This invention relates to waveguide couplers.
Waveguide couplers are employed to transfer electromagnetic radiation between two or more coupled waveguides. For example, optical fibre waveguide couplers couple two or more optical fibres together so that light launched into one fibre is at least partially coupled into the other fibre(s). These techniques are described in the publication xe2x80x9cOptical Waveguide Theoryxe2x80x9d, (Chapman and Hall, London, 1983).
Attempts have been made to combine waveguide couplers with optical fibre gratings to produce wavelength-selective splitting of an optical signal. Such an arrangement would be very useful in splitting, for example, wavelength-division multiplexed optical communication signals, and would replace the present use of bulk (non-fibre) optical devices such as optical circulators. In one form, these grating-couplers use a 50:50 2xc3x972 directional coupler (i.e. one in which half of the light launched into one of two coupled fibres is coupled into the other fibre) with a fibre grating on one of the output ports of the coupler. The fibre grating acts in effect as a wavelength selective mirror, reflecting light at or near to a particular wavelength (referred to as the xe2x80x9cBraggxe2x80x9d wavelength), and transmitting light at other wavelengths. The reflected light passes back into the coupler and is split once again between the two fibres.
The result of this arrangement is that light launched into the first fibre at the Bragg wavelength is reflected back to one of the output ports of the second fibre, but is diminished in intensity by 75% (having passed through the 50:50 coupler twice). The transmitted light also suffers a loss of 50%. In, for example, a communications system involving signal transmission along long lengths of optical fibre, these additional losses impose greater constraints on the length of optical fibre which can be used, the signal to noise ratio of the received optical signal, and/or the number of optical channels which can be propagated along a single fibre.
In another form, as described in the publication entitled xe2x80x9cCompact all-fibre narrowband transmission filter using Bragg gratingsxe2x80x9d (European Conference on Optical Communication, ECOC ""93 post-deadline paper 12.8, page 29, 1993), the grating-couplers use a 50:50 2xc3x972 directional coupler with fibre gratings on each of the output ports of the coupler. The fibre gratings reflect light at or near to the Bragg wavelength, and transmit light at other wavelengths. The reflected light passes back into the coupler arid is recombined in such a way as to re-emerge from the second input port of the coupler. For this to work, the recombination must be interferometrically exact. making the device potentially highly sensitive to environmental changes.
EP-A-0 234 828 discloses a coupler in which a grating disposed between the two fibres causes wavelength-selective contradirectional coupling.
This invention provides an optical fibre coupler comprising at least a first optical fibre coupled at a coupling region to a second optical fibre such that at least a part of radiation propagating along the first optical fibre is coupled into the second optical fibre;
characterised in that the second optical fibre comprises a diffraction grating disposed within the core of the second optical fibre at the coupling region to inhibit coupling of radiation from the first optical fibre into the second optical fibre at wavelengths characteristic of the diffraction grating.
The invention addresses the above problems by providing a wavelength selective coupler in which the coupling from one optical fibre to another can be selectively inhibited by the use of a diffraction grating disposed at the coupling region in the recipient optical fibre. This arrangement can therefore be referred to as a xe2x80x9cgrating frustrated couplerxe2x80x9d. The wavelengths inhibited from coupling can be similar to those lying within the xe2x80x9cstop bandxe2x80x9d of the grating. By increasing the strength of the grating, the inhibition to coupling can be increased. The wavelength selective properties therefore do not depend on multiple passes through an intensity dividing coupler, and so the overall power losses involved in selecting a particular wavelength range can be reduced.
In a preferred embodiment, substantially all of the radiation is coupled into the second optical fibre at wavelengths other than the wavelengths characteristic of the diffraction grating. This then provides an efficient channel-dropping filter, in which the characteristic wavelengths remain in the first optical fibre, and the remainder of the radiation is coupled into the second optical fibre. The power loss suffered by the remainder of the radiation is thus very low.
In order to equalise any disruption to the propagation through the two optical fibres caused by the presence of the diffraction grating in the second optical fibre, in a preferred embodiment the first optical fibre comprises a diffraction grating disposed at the coupling region, the characteristic wavelengths of the diffraction grating in the first optical fibre being different to the characteristic wavelengths of the diffraction grating in the second optical fibre. The diffraction grating in the first optical fibre can have characteristic frequencies which are far removed from the wavelengths of interest, and thus can serve merely to provide similar propagation in the two optical fibres (which in turn can improve the coupling efficiency). Alternatively, in embodiments of the invention the first optical fibre is uniform or substantially uniform (i.e. it does not have a grating).
Improved wavelength selection performance can be obtained in preferred embodiments in which the diffraction grating in the second optical fibre extends along the second optical fibre beyond the coupling region. It is further preferred that the diffraction grating in the second optical fibre is substantially twice as long as the coupling region.
It is preferred that the diffraction grating in the second optical fibre comprises a periodic modulation of the refractive index of the second optical fibre. Because some techniques for the fabrication of such periodic modulations also tend to raise the spatially averaged refractive index of the fibre, it is preferred that this is compensated by the first optical fibre (or at least the coupling region of the first optical fibre) having a higher refractive index than the second optical fibre.
A coupler according to the invention is particularly advantageously employed in communication apparatus.